The invention relates to altimeters, and more particularly to a solid state altimeter device sufficiently compact to be worn on the wrist and including additional features in conjunction with the altimeter function.
Various forms of altimeters are well known, particularly those for use in aircraft. Conventionally, altimeters have been mechanical in nature, essentially comprising an atmospheric pressure-responsive bellows connected mechanically to an indicator needle. Other altimeters have used a metal disk with strain gauges bonded to the disk.
More recently, there have been developed special solid sate pressure sensors or transducers, of smaller size than the conventional bellows. In these pressure transducers a strain-sensitive element or strain gauge has been encapsulated directly within a silicon member which acts as a diaphragm. Such transducers have been used to measure pressure differential between the two sides of the diaphragm.
For example, U.S. Pat. No. 4,303,903 discloses a solid state pressure transducer, not forming a part of an altimeter, but generally of the type employed in the present invention described below.
An altimeter showing the use of generally this type of solid state transducer is disclosed in U.S. Pat. No. 4,106,343. That altimeter, designed for aircraft use, included display of altitude, correction for barometric pressure, and a form of rate of climb indicator consisting of an array of LEDs.
U.S. Pat. No. 4,030,065 discloses an aircraft terrain clearance warning system having an automatic voice warning tied in with a radio altimeter.
Piezoresistance of a semiconductor can be described as the change in resistance caused by an applied strain of the diaphragm. Thus, solid state resistors can be used as pressure sensors much like wire strain gauges, but with seveal important differences and advantages.
High sensitivity, or gauge factor, in a semiconductor strain gauge is perhaps 100 times that of wire strain gauges. Piezoresistors are implanted into a homogeneous single cyrstalline silicon medium. The implanted resistors are thus part of the silicon force sensing member.
Prior conventional strain gauges typically were bonded to members of dissimilar material, resulting in thermo-elastic strain and complex fabrication processes. Most strain gauges are inherently unstable due to degradation of this bond, as well as temperature sensitivity and hysteresis caused by the thermo-elastic strain.
Silicon is an ideal material for receiving the applied differential pressure force. As a perfect crystal it does not contribute to hysteresis error. After being strained, it returns to the original shape.
Prior mechanical altimeters have been costly as compared with the present invention, and yet still have not performed all of the functions of the invention, usually being limited to reporting altitude and sometimes rate of climb.
Although some of the prior art included transducers or singular features relating to the present invention, none has disclosed or suggested an altimeter device of the compact design and having the advantageous features of the present invention, as described below.